diff --git a/modp.py b/modp.py
index 19b15d9..18a626c 100644
--- a/modp.py
+++ b/modp.py
@@ -24,4 +24,17 @@ class ModP(int):
     
     def _inv(self):
         'Find multiplicative inverse of self in Z mod p.'
-        pass
\ No newline at end of file
+        # extended Euclidean algorithm
+        rcurr = self.p
+        rnext = int(self)
+        tcurr = 0
+        tnext = 1
+        
+        while rnext:
+            q = rcurr // rnext
+            rcurr, rnext = rnext, rcurr - q * rnext
+            tcurr, tnext = tnext, tcurr - q * tnext
+        
+        if rcurr != 1:
+            raise ValueError("%d not a unit modulo %d" % (self, self.p))
+        return ModP(self.p, tcurr)
\ No newline at end of file
diff --git a/padic.py b/padic.py
index a79dd75..bdbbf6e 100644
--- a/padic.py
+++ b/padic.py
@@ -31,8 +31,6 @@ class PAdic:
         return int(self.get(32), p)
     def __str__(self):
         return self.get(32)
-    def __repr__(self):
-        pass
     
     # arithmetic operations
     def __add__(self, other):